In the manufacturing industry, there is a great need for high precision instruments which are capable of measuring distances within exacting tolerances. Even if a machine is able to measure and move within precise limits, it is desirable for the initial position of the measuring apparatus to be consistent from day to day. If a machine measures distance with respect to a home position, the accuracy of the machine will depend upon the accuracy of the home position.
Accordingly, the linear variable differential transformer (LVDT), which is capable of measuring extremely small changes in distance, has been extensively used in measurement instrumentation. Unfortunately, the LVDT, which is relatively inexpensive, presents certain limitations which prevent its extensive use within high quality measurement apparatus. For example, the cores within typical LVDTs are usually permanently positioned within the chamber of the LVDT, preventing the core from leaving the chamber. Therefore, the measuring apparatus which is mechanically connected to the core is not able to travel distances beyond that afforded by the LVDT. If the core is able to exit the chamber of the LVDT, the measuring apparatus must take into account the presence of stray signals within the secondary windings even when the core is absent. Due to manufacturing tolerances, these stray signals may vary in both magnitude and polarity. Although the manufacturing tolerances may be controlled, such control is quite expensive.
Accordingly, there is a need for a measuring apparatus which can use inexpensive LVDT's in a highly precise measuring instrument.